The field of the invention comprises apparatus and methods for imaging electrophotographic members by means of radiant energy devices such as lasers, the imaged electrophotographic members being thereafter used for printing. In the case of lithographic offset printing, the imaged member itself is treated to render toned and untoned parts hydrophobic and hydrophilic, respectively, and the member comprises the printing plate without further processing. In other cases, the toned electrophotographic member may be used as an information source by reading the images or projecting them if transparent or photographically reproducing them if desired. The preferred use of the invention is to make printing plates upon metal substrates such as stainless steel. These substrates are coated with a type of photoconductive coating which will be described hereinbelow.
In the printing industry, printing plates for printing both graphics and text have in the past been produced manually with the graphics images being reproduced using the so-called half-tone process. In this process several photographic steps are used to reproduce the graphics image in an array of dots of varying size to reproduce the image on the printing plate. Text information has in the past been hand set, but now may be set by machine under control of electronic devices.
Forming printing plates carrying both graphics and text images may involve several steps, especially when color graphics are to be reproduced. In such a case, several color separation plates must be made for each color to be printed with the text information being carried on the plate in which color the text is to be printed. When text information is to be located within the field of the graphics image, additional steps are required to form the solid printing text areas for the plates in that particular color and to remove the graphics image from those same text areas on the remainder of the color separation plates. This of course adds to the number of process steps required to produce the desired graphics and text images. The steps of forming the text image to be printed in the graphics field is commonly known as overburning while the process of removing the graphics image from those same text areas in the other color separation plates to be printed is referred to or is commonly known as stripping.
In overburning, the negatives which form the graphics image and the text image to be formed in that field are overlayed one on another to form the desired color separation printing plate. In stripping, other techniques must be used to remove the graphics information from those same text image areas.
The process of forming printing plates contained both graphics and text data recently has been implemented electronically using essentially the same methods as were performed manually. Advanced systems however are able to complie data from various input devices that may be used to form both graphics and text information on a printing plate. These systems have their drawbacks in that separate scanning cycles must be performed to form the graphics and text images on a single printing plate and in addition, complex switching circuits must be constructed to switch between text and graphics image formation when text images are to be formed within the field of a graphics image.
The apparatus and method of the present invention overcome the drawbacks presented by the manual and previous electronic systems by providing a system in which one pass of a beam of radiant energy may form both graphics and text images in response to graphics and text data input thereto. Formation of the graphics and text images may occur independently of one another so that different imaging schemes may be used to form scaled densities of the graphics images and the binary densities of the text images.
Formatting of the data in accordance with the invention is such that the graphics data contains information related to the relative scaled densities of incremental areas of the graphics image with the remainder of the graphics data for the remainder of the printing plate being a nullity to clear the surface of the charged electrophotographic member. The text data is formated such that it does not affect the formation of the images carried by the graphics data except in locations where text images are to be formed.
Formation of text images within the field of graphics images for several different color separation plates is performed simply by reversing the logical sense of a control bit of every text data digital word. Thus, to produce text images of one color such as blue in the field of a multicolor printed graphics image, the same text data may be used for all of the color separation plates with the control bit for the color separation plate used to print the color blue being set to one logical state and being set to the other logical state for the remainder of the color separation plates.
Thus the apparatus and method of the invention provide for imaging of an entire printing plate with graphics and text information in a single pass of a beam of radiant energy.
The apparatus and method of the invention include an optical system in which a beam of radiant energy from a monochromatic source such as a laser is used selectively to discharge and to leave charged incremental areas of a charged electrophotographic member. Part of the beam is split therefrom and is used as a reference beam. The remainder of the beam is modulated to provide a scanning beam or a fine beam comprised of individual rays of radiant energy with each ray able to discharge an incremental area of the member. The reference beam and scanning beam or fine beam are aligned vertically with one another with the vertical alignment being used in an optical grating system precisely to determine the location of the scanning beam along the surface of the member. A field flattening lens is used in which both the reference and fine beams are passed through and back again to the member, the field flattening lens providing the maintenance of a focused image on the surface of the member across every scan line.
A common technique to determine the instantaneous position of the scanning beam along a scan line of the member is to employ an optical scale or grating composed of alternate bars or spaces of opaque and transparent, or absorbing and reflecting surfaces or areas. These alternating spaces occur at intervals equal to the spacing between formable elements on the member to provide electrical signals indicating the alignment of the scanning beam with the elements. Light passing through or being reflected from such a grating is detected with a photosensitive device which converts the detected energy into electrical pulses.
Over relatively short scan widths, say 10 inches or so, the problem of accurately gathering or collecting light pulses from an optical scale and directing them to the photosensor is readily accomplished with relatively simple optics. In much greater scan widths however the cost of collecting optics rises exponentially and quickly reaches prohibitive levels. The apparatus of the invention herein has an active scan length of preferably 24 inches. The cost of conventional optics for collecting a reference beam across such a length and establishing a beam feed-back signal within 1/300 of an inch accuracy is prohibitive.
The concept of using a glass rod or fiber in such a grating collection system is known. The principle used involves having the beam strike the rod perpendicular to its cylindrical surface to collect the intercepted energy in the rod and detect the intercepted energy as it exits the rod at either end thereof. Original results with a short piece of 3/8 inch diameter glass rod provided poor results, it being believed that most of the energy from the beam was transmitted through the diameter of the rod so that the light output at either end of the rod was too low to be of use.
The concept of using a hollow metal tube with a high reflective interior surface to reduce transmissive losses also was investigated. The tube used had a very narrow length-wise slit to provide for entrance of the radiant energy reference beam, and a photosensor was mounted at one end of the tube with a mirror located at the other to reinforce the reflected energy levels. It was believed that the reference beam would strike the rear internal surface of the tube and give rise to multiple reflections which would propagate along the tube and result in a useful output level at the end mounted sensor. The optical surface smoothness on the interior was difficult to control and in turn unsatisfactory reflections and distributions were obtained. At a consequency thereof, signal levels obtained from the hollow metal tube varied greatly as a function of the beam position from the sensor along the scan length. Automatic gain and compensation techniques were implemented to modulate the electronic signal from the sensor, but none of these proved successful. In reevaluating the glass rod technique, it was believed that if the transmissive losses of energy could be prevented by containing the light within the fiber as within the hollow tube, the rod collecting scheme might succeed.
A 13/4 inch rod was used because the internal diameter of the existing hollow tube was about 2 inches and this would facilitate concentric mounting of the rod within the tube, and would further minimize energy losses by decreasing the concentric area. Essentially, the glass fiber rod was mounted within the length of the tube. Initial tests met with little success until a strip of masking tape was attached longitudinally of the rod opposite the beam entry point. Increased energy level from the non-reflecting surface of the tape was immediately recognized to be the result of eliminating the air-gap index of refraction (a high loss component) while containing and reflecting the entrapped energy in the rod. It was quickly determined that highly reflective material such as a typewriter corrector fluid applied to the rod's cylindrical surface would be highly efficient in preventing the transmissive loss and aid in providing good Lambertian distributions. It was later determined that it was not necessary to coat the entire surface of the cylinder or rod. A narrow stripe about 1/4 of an inch wide along the rod proved to be more than adequate. Test results for rods of 0.78 inch, 1.0 inch, 1.5 inch and 1.75 inch diameter indicated that the best results for the bar collector used in the present invention would be obtained with a bar diameter somewhere between 1.5 inch and 1.75 inch.
Two prior art patents which disclose using a bar collector in an optical scanning or sensing apparatus are U.S. Pat. Nos. 4,040,748 and 4,040,745. These patents however do not appear to disclose the use of a bar collector over the length provided by the invention herein.
The apparatus and method of the invention further include an electronic system that controls the graphics and text imaging process of the invention. As has been explained, the invention provides for the intermixed formation of graphics and text images on the electrophotographic member in one sweep or pass of the imaging beam of radiant energy. The electronics provided are such that formation of the graphics and text images occur independently of one another, i.e., both graphics and text images may be formed at any location of the member.
The electrophotographic member used with the apparatus and method of the invention proives incremental areas to be imaged that are finer than are presently available and provides that those elements may be formed at a more rapid rate and with less energy than has previously been provided for. This electrophotographic coating will be further referred to hereinafter and is the coating described and claimed in U.S. Pat. No. 4,025,339.
The apparatus and method of the invention further include a toning system which applies minute toning particles to the areas of the latent image that remain charged. This toning system provides an essentially vertical meniscus closely spaced from the horizontal line at which imaging of the member occurs so that there is a minimal loss of voltage, representing the latent image on the electrophotographic member, from imaging to toning. Toning systems are known in which toning fluid is applied to the bottom of a rotating drum carrying the electrophotographic member wherein the distance from the imaging to the toning is minimal. In the apparatus of the present invention however, a large drum is used which rotates relatively slowly so that if a toning system were used that is located at the bottom of the drum, substantially all of the latent image would become discharged by the time the member was rotated to the toning station. Therefore, the toning station must be located closely spaced from the plane or horizontal line at which imaging occurs, which requires that toning fluid be applied in a layer which is essentially vertical.
This vertical layer or meniscus is provided by a supply or pressure system sealed to the atmosphere, providing toning fluid to escape therefrom in the form of the layer or meniscus of toning fluid. The rate of escape of the toning fluid is controlled by a valve admitting atmosphere to the otherwise sealed pressure system so that the rate of flow of the toning fluid from the system is substantially equal to the movement of the member past the toning station to provide a vertical meniscus of toning fluid that is substantially stationary relative to the member.